Underwater remote sensing

ABSTRACT

An underwater communications system is provided that transmits electromagnetic and/or magnetic signals to a remote receiver. The transmitter includes a data input. A digital data compressor compresses data to be transmitted. A modulator modulates compressed data onto a carrier signal. An electrically insulated, magnetic coupled antenna transmits the compressed, modulated signals. The receiver that has an electrically insulated, magnetic coupled antenna for receiving a compressed, modulated signal. A demodulator is provided for demodulating the signal to reveal compressed data. A de-compressor de-compresses the data. An appropriate human interface is provided to present transmitted data into text/audio/visible form. Similarly, the transmit system comprises appropriate audio/visual/text entry mechanisms.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 11/454,630 filed Jun. 15,2006, which application claims the benefit of Ser. No. 60/690,964,60/690,966, and 60/690,959, all filed Jun. 15, 2005, and is acontinuation in part of GB0602398.0, filed Feb. 7, 2006, all of whichapplications are fully incorporated herein by reference.

BACKGROUND

1. Field of Use

The present invention relates generally to an underwater communicationssystem, and its methods of use, and more particularly to an underwatercommunications system that uses electromagnetic propagation and magneticinduction transmission, and optimizes the distance which can be achievedby digital transmission of information.

2. Description of the Related Art

Various underwater communication systems are known. One of the mostcommon is based on acoustic techniques. A problem with such systems isthat they are degraded by noise and interference from a number ofsources. They are also subject to multi-path effects and in someenvironments are virtually unusable. Other underwater communicationsystems use radio links, e.g. extreme low frequency electromagneticsignals, usually for long-range communications between a surface stationand a submerged vessel. These systems typically operate in the far fieldusing physically large electric field coupled antennas and support datarates up to a few bits per second.

WO01/95529 describes an underwater communications system that useselectromagnetic signal transmission. This system has a transmitter and areceiver, each having a metallic, magnetic coupled aerial surrounded bya waterproof electrically insulating material. Use of electricallyinsulated magnetic coupled antennas in the system of WO01/95529 providesvarious advantages. This is because magnetically coupled antennas launcha predominantly magnetic field. A similar arrangement is described inGB2163029. Whilst the communications systems of WO01/95529 and GB2163029have some technical advantages over more conventional acoustic or radiolink systems, the functionality described is limited, and for manypractical applications the available bandwidth is highly restrictive, asis distance over which data can be transmitted.

Magnetic antennas formed by a wire loop, coil or similar arrangementscreate both magnetic and electromagnetic fields. The magnetic ormagneto-inductive field is generally considered to comprise twocomponents of different magnitude that, along with other factors,attenuate with distance (r), at rates proportional to 1/r² and 1/r³respectively. Together they are often termed the near field components.The electromagnetic field has a still different magnitude and, alongwith other factors, attenuates with distance at a rate proportional to1/r. It is often termed the far field or propagating component.

Signals based on electrical and magnetic fields are rapidly attenuatedin water due to its partially electrically conductive nature. Seawateris more conductive than fresh water and produces higher attenuation.Propagating radio or electromagnetic waves are a result of aninteraction between the electric and magnetic fields. The highconductivity of seawater attenuates the electric field. Water has amagnetic permeability close to that of free space so that a purelymagnetic field is relatively unaffected by this medium. However, forpropagating electromagnetic waves the energy is continually cyclingbetween magnetic and electric field and this results in attenuation ofpropagating waves due to conduction losses.

The attenuation losses, the bandwidth restrictions and the limiteddistances over which data can be transmitted all pose significantpractical problems for underwater communications.

Existing methods of acoustic communication are inherently restricted inthe distance they can achieve at effective data rates. This isparticularly true where the signal reaches a receiver by multiple paths(reflections occurring from an irregular sea floor, the sea surface, thecoastline, nearby objects and the like, we well as when the sound wavepath exhibits discontinuities in its properties (wave wash, bubbles inthe water, changes in water density due to salinity variations). Littleis known which can lessen these difficulties. The existing art ofelectromagnetic communication under water fails to recognize measuresthat can be taken to maximize the distance and/or useful informationrate which can be achieved by adapting the devices sourcing and usingthe information so that more effective signal frequencies can beadopted.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide animproved underwater communication systems, and its methods of use, thatuses electromagnetic waves for communication and propagation.

Another object of the present invention an underwater communicationsystem, and its methods of use, for communication and propagation thatincreases the distance over which information can be transmitted.

Another object of the present invention an underwater communicationsystems, and its methods of use, for communication and propagation thatincreases the useful information rate.

Another object of the present invention an underwater communicationsystems, and its methods of use, for communication and propagation withimproved data compression by reducing the transmitted bit rate.

Another object of the present invention an underwater communicationsystems, and its methods of use, for communication and propagation wherethe transmitted bit rate is reduced when there are a number of types ofinformation sources.

Another object of the present invention an underwater communicationsystems, and its methods of use, for communication and propagation thathas a resultant reduced bit rate that allows lower transmitted signalfrequencies to be adopted.

Another object of the present invention an underwater communicationsystem, and its methods of use, for communication and propagation thathas lower transmitted signal frequencies to achieve greater distanceand/or allow greater rates at a particular distance.

These and other objects of the present invention are achieved in, anunderwater communications system for transmitting electromagnetic and/ormagnetic signals to a remote receiver that includes a data input. Adigital data compressor compresses data to be transmitted. A modulatormodulates compressed data onto a carrier signal. An electricallyinsulated, magnetic coupled antenna transmits the compressed, modulatedsignals.

In another embodiment of the present invention, an underwatercommunications system includes a receiver that has an electricallyinsulated, magnetic coupled antenna for receiving a compressed,modulated signal. A demodulator is provided for demodulating the signalto reveal compressed data. A de-compressor de-compresses the data.

In another embodiment of the present invention, an underwatercommunications system includes a transmitter for transmittingelectromagnetic and/or magnetic signals. A receiver receives signalsfrom the transmitter. At least one intermediate transceiver receiveselectromagnetic and/or magnetic signals from the transmitter and passesthem to the receiver. At least one of the transmitter and receiver isunderwater and includes an electrically insulated, magnetic coupledantenna.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of one embodiment of an underwater transceiverof the present invention.

FIG. 2 shows the attenuation typically experienced by a signalpropagating in seawater, and how its attenuation expressed in decibelsincreases approximately in proportion to the square root of frequency.

FIG. 3 is a block diagram of an underwater transmitter that can be usedin the transceiver of FIG. 1.

FIG. 4 is a block diagram of an underwater receiver for use in thetransceiver of FIG. 1.

FIG. 5 is a block diagram of an underwater antenna for use in thetransmitter of FIG. 3 and receiver of FIG. 4.

FIG. 6 is a block diagram of one embodiment of a system of the presentinvention for allowing underwater diver-to-diver voice communications.

FIG. 7 is a block diagram of one embodiment of a system of the presentinvention for allowing underwater diver-to-diver text communications.

FIG. 8 is a block diagram of one embodiment of a system of the presentinvention for reduced bandwidth voice communications using speechrecognition and speech synthesis techniques to allow narrow bandwidthtext transmission.

FIG. 9 is a block diagram of one embodiment of a system of the presentinvention for allowing underwater video communications;.

FIG. 10 is a block diagram of one embodiment of a mesh network of thepresent invention for use in an underwater communications system;

FIG. 11 is a block diagram of one embodiment of a water to surfacecommunication system of the present invention.

FIG. 12 is a block diagram of one embodiment of a communication systemof the present invention that can be used for an autonomous underwatervehicle.

FIG. 13 illustrates a modulation signal for use in a signal modulationscheme.

FIG. 14 illustrates a transmission scheme that employs guard bands.

FIG. 15 is a schematic view of one embodiment of a communications systemof the present invention that is operable above water and below water,as well as when the system is awash.

DETAILED DESCRIPTION

In various embodiments, the present invention relates to underwatercommunication systems, and their methods of use, that useelectromagnetic signals as the communicating means. Each of these uses acommunications transceiver 10 that has a transmitter 12, a receiver 14and a processor 16 which can be connected to an analogue or digital datainterface (not shown), as illustrated in FIG. 1. Both the transmitterand receiver 12 and 14 respectively have a waterproof, electricallyinsulated magnetic coupled antenna 18 and 20. Alternatively a singleantenna can be shared between transmitter and receiver. A magneticcoupled antenna is used because water is an electrically conductingmedium, and so has a significant impact on the propagation ofelectromagnetic signals. Ideally, each insulated antenna assembly issurrounded by a low conductivity medium that is impedance matched to thepropagation medium, for example distilled water.

Electrically insulated magnetic coupled antennas are used in thecommunication systems in which various embodiments of the presentinvention are embodied because in an underwater environment they aremore efficient than electrically coupled antennas. Underwaterattenuation is largely due to the effect of conduction on the electricfield. Since electrically coupled antennas produce a higher electricfield component, in water in the near field, the radiated signalexperiences higher attenuation. In comparison a magnetic loop antennaproduces strong magneto-inductive field terms in addition to theelectromagnetic propagating field. The magneto-inductive terms aregreater than the propagating field close to the transmitting antenna andprovide an additional means for coupling a signal between two antennas.For both shorter and greater distances, magnetic coupled antennas aremore efficient under water than electrically coupled. In applicationswhere long distance transmission is required, the magnetic antennashould preferably be used at lowest achievable signal frequency. This isbecause signal attenuation in water increases as a function ofincreasing frequency, as shown in FIG. 2. Hence, minimizing the carrierfrequency where possible allows the transmission distance to bemaximized. In practice, the lowest achievable signal frequency will be afunction of the desired bit rate and the required distance oftransmission.

FIG. 3 illustrates an example of a transmitter 12 for use in thetransceiver 10 of FIG. 1. This has a data interface 22 that is connectedto each of a processor 24 and a modulator 26. The modulator 26 isprovided to encode data onto a carrier wave. At an output of themodulator 26 are a frequency synthesiser 28 that provides a localoscillator signal for up-conversion of the modulated carrier and atransmit amplifier 30, which is connected to the underwater,electrically insulated magnetic coupled antenna 18. In use, thetransmitter processor 24 is operable to cause electromagneticcommunication signals to be transmitted via the antenna at a selectedcarrier frequency.

FIG. 4 illustrates an example of a receiver 14 for use with thetransceiver of FIG. 1. As with the transmitter, this has an electricallyinsulated magnetic antenna 20 adapted for underwater usage. This antennais operable to receive magnetic field signals from the transmitter.Connected to the antenna 20 is a tuned filter 32 that is in turnconnected to a receive amplifier 34. At the output of the amplifier 34is a signal amplitude measurement module 36 that is coupled to ade-modulator 38 and a frequency synthesiser 40, which provides a localoscillator signal for down conversion of the modulated carrier.Connected to the de-modulator 38 are a processor 42 and a data interface44, which is also connected to the processor 42. The data interface 44is provided for transferring data from the receiver 14 to a control ormonitoring means, such as another on-board processor, which may belocated in the mobile device 10 or at another remote location.

As an alternative, or additional, type of receiver to that of theheterodyne method described, communications practioners will appreciatethat a receiver of the homodyne principle may be employed.

In one embodiment of the present invention, an underwater communicationssystem is provided for transmitting data to a remote receiver. In thisembodiment, the system can have a data input; a data compressor forcompressing data that is to be transmitted; a modulator for modulatingthe compressed data onto a carrier signal and an electrically insulated,magnetic coupled antenna for transmitting the compressed, modulatedsignals. It will be appreciated that the words remote and local usedherein are relative terms used merely to differentiate device sites forthe purpose of description, and do not necessarily imply any particulardistances.

By compressing the data prior to transmission, the occupied transmissionbandwidth can be reduced. This allows use of a lower carrier frequency,which leads to lower attenuation. This in turn allows communication overgreater transmission distances, thereby significantly alleviating thedifficulty of communication through water. Digital representation ofaudio and or video, data compression and transmission at the lowestpracticable frequency are therefore particularly advantageous in thesubsea environment and represents a key innovation. While datacompression is usually highly desirable, it will be appreciated that itis not essential to the operation of different embodiments of thepresent invention.

Whether or not compressed, data in some applications of the presentinvention can be encrypted before transmission and decrypted afterreceiving, when desired for reasons of security. Although a low carrierfrequency is usually optimal to maximize distance, there may beoccasions when a higher frequency is satisfactory but more desirable inorder to reduce the distance over which an unwanted receiving party candetect the signal, as in deliberately covert operation of acommunication system.

In one embodiment of the present invention, error correction techniquesare applied to the information transferred. Error correction techniquesslightly increase the amount of data which must pass over thecommunication links themselves, but can be advantageous in allowingoperation at greater distances which otherwise would have resulted inunreliable transfer of information. Error correction can be of the typescommonly and generically known as forward error correction (FEC) andautomatic repeat request (ARQ). For somewhat random errors which arewell spaced and do not occur in long runs, FEC is preferable; andbeneficially the effectiveness of FEC may be increased by first applyingan interleaving process, as known in the art.

In various embodiments, the system of the present invention can includedata/text entry means, such as a keypad, and/or audio means forcapturing audio signals and/or video means, such as a camera, forcapturing an image. Having inputs, such as a text entry pad and acamera, provides an extended functionality device, and extends the rangeof device applications. Alternative approaches such as employing speechto text conversion and text to speech offer additional bandwidthreduction and therefore range benefits.

In one embodiment, a display may be provided so that text/data enteredand/or video/images can be viewed prior to transmission.

In one embodiment, the communications module of the present inventionincludes a receiver that has an electrically insulated, magnetic coupledantenna for receiving electromagnetic signals. In this embodiment, themodule is preferably operable to present received text/data and/orvideo/images on the module display. The transmitter and the receiver mayshare a single electrically insulated, magnetic coupled antenna.

In one embodiment, the system of the present invention can be configuredto change the carrier frequency to optimise the informationcommunication rate for the transmission range and conditionsencountered. In another embodiment, the system of the present inventioncan be configured to establish a connection; commence transmission at afirst frequency; once communication is established, vary the frequencyand select the frequency based on the received signal strength.

In one embodiment of the present invention, the magnetic coupled antennaused with certain embodiments of the present invention can be based onloops or solenoids. The solenoid may be formed around a high magneticpermeability material. The insulated antenna may be surrounded with alow conductivity material with permittivity matched to that of thepropagation medium e.g. distilled water.

To further improve communications, the transmission distance has to betaken into account. By way of illustration, for short distancetransmission, the magnetic components provide the greater signal,whereas longer distances are best served by the electromagneticcomponent. Hence for short distance communications, near fieldtransmission is preferred, whereas for longer distance communications,far field transmission is preferred. Whether the magnetic components orthe electromagnetic component dominate is a matter dependent on theapplication of the invention and the distance over which it is deployed.

In another embodiment of the present invention, an underwatercommunications system is provided that includes, an underwatertransmitter having an electrically insulated, magnetic coupled antennafor transmitting electromagnetic signals to a receiver, and anunderwater receiver having an electrically insulated, magnetic coupledantenna for receiving signals from the underwater transmitter, whereinthe transmitter and receiver are adapted to communicate when the nearfields of the transmitting antenna and receiving antenna overlap. Thenear field may be defined approximately as the region where the 1/r² and1/r³ varying terms are greater than the propagating 1/r term (wherer=radial distance).

Near field subsea magneto-inductive communications links can supportmuch higher carrier frequencies than possible in the far field. In turn,communication in the near field allows a significantly higher signalbandwidth than is available for far field transmissions. While the nearfield components are relatively greatest close to an antenna, their rateof decline with distance is faster than that of the far field component.When the antenna is magnetic, the important advantage of lower loss isgained over conventional electromagnetic antennas of the types commonlyused in free space. In addition the relative initial strength of themagnetic field in comparison with the electromagnetic field isconsiderably greater still.

In another embodiment of the present invention, an underwatercommunications system includes a transmitter for transmittingelectromagnetic signals to a remote receiver, and a receiver forreceiving signals from the transmitter, wherein at least one of thetransmitter and receiver is underwater and has an electricallyinsulated, magnetic coupled antenna. One of the transmitter and receivermay be above water and may have an electrically coupled antenna.

In another embodiment of the present invention, an underwatercommunications system includes, a transmitter for transmittingelectromagnetic signals to a remote receiver, and a receiver forreceiving signals from the transmitter, wherein at least one of thetransmitter and receiver includes means for varying the signal gain.This is advantageous for systems in which one or both antennas may besubjected to wave wash, where the antenna is periodically partially orwholly immersed in water. By providing means for varying the gain,performance can be maintained even when one or more of the antennas issubject to wave wash.

In another embodiment of the present invention, an underwatercommunications system includes a device for transmitting electromagneticsignals and means for transmitting acoustic signals and/or opticalsignals. In use, the system of this embodiment can be\controlled suchthat the optimal route for communication is utilized be itelectromagnetic, acoustic or optical. Under different or changingconditions, one or more of these methods may provide superiorperformance at different times.

For reception of weak signals, such as at greater distances, thereduction of received interfering noise will be important. This may beaccomplished by filtering the received signal to the minimum bandwidthpossible, consistent with the bandwidth of the wanted signal, beforemaking decisions on the received digital signal states. Alternatively,or in addition, digital bit states may be represented in transmission byknown and readily distinguishable sequences of sub-bits transmitted at ahigher rate, and correlation techniques adopted to determine the likelypresence of each sequence and hence the value of each received bit. Suchtechniques will be familiar to those skilled in the techniques ofcommunication in other fields.

A further technique, often advantageous where effects such as multi-pathpropagation, fading and dispersion exist between transmitter andreceiver, is that of spread spectrum, in which transmission power isdeliberately distributed over a wide bandwidth and correlation methodsare used in receivers. As will be known to communication practioners,the spread spectrum technique is enhanced if the known RAKE method isalso adopted in receivers.

Furthermore, while carrier-based techniques with impressed modulationhave been described, un-modulated methods without a carrier also may beadopted, wherein a representation of the baseband data is used directlyto energize the antenna.

FIG. 5 illustrates an example of an antenna that can be used in thetransmitter and receiver of FIGS. 3 and 4. This has a high permeabilityferrite core 46. Wound round the core are multiple loops 48 of aninsulated wire. The number of turns of the wire and length to diameterratio of the core 46 can be selected depending on the application.However, for operation at 125 kHz, one thousand turns and a 10:1 lengthto diameter ratio is suitable. The antenna is connected to the relevanttransmitter/receiver assembly and is included in a waterproof housing50. Within the housing the antenna may be surrounded by air or someother suitable insulator 52, for example, low conductivity medium suchas distilled water that is impedance matched to the propagating medium.

FIG. 6 illustrates a telecommunications system 54 for underwaterdiver-to-diver or similar communications. This includes twotelecommunications units 56 and 58, each of which includes thetransceiver of FIG. 1. In this case, connected to the processor 16 ofthe transceiver is an acoustic interface 60, which is connected betweena speaker 62 and a microphone 64. The processor 16 is operable to allowvoice-to-voice communication between divers using electromagneticpropagation as the communication channel. One function of the processor16 is to convert the digital representation of analogue signals from theacoustic interface 60 to and from a highly compressed form of encoding,using known vocoder or similar techniques which greatly reduce therequired bit-rate for intelligible conversation. The transmissionprotocol used could be point-to-point or broadcast, depending on theapplication. In practice, the speech compression algorithms used toreduce bandwidth and carrier frequency significantly increases the rangeof reception for underwater communications. In this case the carrierfrequency could typically be in the range 1-100 kHz, for example 30 kHz.Techniques for allowing wireless voice-to-voice communications are knownin the art and so will not be described in detail, save to say thatmodern vocoder techniques can achieve very low bit-rates while stillensuring good intelligibility between persons. By using such a vocoder,the low bit rates can enable greater distance to be achieved underwater. In practice, the units would be included in a full-face mask, ascommonly used by commercial divers, to facilitate voice communications.The speaker would form a waterproof earpiece.

FIG. 7 illustrates another telecommunications system for underwaterdiver-to-diver or similar communications that also has twotelecommunications units 66, 68, each of which includes the transceiverof FIG. 1. In this case, connected to the processor 16 of thetransceiver is a text input 70 for allowing a user to input textmessages and a display 72 for displaying such text messages. Textentered by the users is passed to the on-board processor 16 and then tothe transmitter, where it is digitally modulated onto a carrierfrequency and transmitted. As before, the transmission protocol usedcould be point-to-point or broadcast, depending on the application.Techniques for allowing mobile text communications are known in the artand so will not be described in detail. The devices of FIG. 7 allow textcommunication between divers using electromagnetic propagation as thecommunication channel. Because texting is a very low data rateapplication, a low frequency carrier can be used which will experience acomparatively low attenuation. Signal reception should be possible inthe far field. To maximize range, the carrier frequency used typicallywould be below 100 Hz, for example 40 Hz.

FIG. 8 illustrates an alternative voice communications system 74 forunderwater diver to diver or diver to surface communications. This alsohas two telecommunications units here drawn as separate transmitter andreceiver units 76 and 78 respectively. The transmitter unit has amicrophone 80 that is located, for example, within a diver's mask sothat it is close to the diver's mouth in use. This is connected to aspeech recognition module 82, which is operable to convert speech to anelectronic representation of text. This can be done using commonlyavailable speech recognition software, the output of which is a streamof characters in electronic form. Connected to the speech recognitionmodule 82 is a text encoder 84 that is connected to a transmitter 86that is coupled to a magnetic antenna 88. The encoder 84 converts thespeech signals into text-based data. An example of a suitable format forthis data may be ASCII or preferably a more efficient coding mechanismto minimize the data to be transmitted and therefore the requiredbandwidth. This text-based data is then modulated and transmitted by thetransmitter and antenna 86 and 88 respectively to the antenna 90 andreceiver 92. Connected to the receiver 92 is a demodulator 94, which isoperable to recover the text data. This is processed using a speechsynthesis module 96 to convert the data into artificial voice signalsfor the receiving party to hear. This speech signal is then fed to thereceiving party using a suitable earphone, loudspeaker or other suchdevice 98 as appropriate for the receiving party.

The system of FIG. 8 is particularly beneficial since it reduces databandwidth to the lowest practicable minimum. This allows use of a lowercarrier frequency, and hence a much lower signal attenuation. Bandwidthis much smaller than alternative audio compression techniques, whichmust act to efficiently represent the full range of audio frequenciesthat comprise the human voice. In comparison, a recognition andsynthesis based system only conveys the textual meaning of the speech.The system has the added advantage of being more clearly intelligiblethan a direct audio link. Restrictive facemasks and breathing apparatusaffect speech underwater. A speech recognition algorithm can be adaptedto work effectively with underwater speech. The synthesized speechre-created at the receiver will be of a much higher quality than theoriginal.

FIG. 9 illustrates a telecommunications system for providing a videolink. As before, the system has two telecommunications units 100 and102, each of which includes the transceiver of FIG. 1. However, in thiscase, a camera 104 is provided. The camera generates a digital formatrepresentation of image or image and sound. This is connected via amodem 106 to image compression means 108, which is connected in turn tothe transceiver processor 110. Connected to the processor 112 of theother transceiver are a corresponding image decompression means 114, amodem 116 and a display 118. Images captured by the first transceiver100 can be sent to the second transceiver 102 using near field or farfield electromagnetic propagation as the communication channel, andsubsequently displayed on the display. Video compression algorithms areapplied to reduce the bandwidth and carrier frequency, therebyincreasing the range of reception for underwater communications. In thiscase the carrier frequency would typically be in the range 100-200 kHz,for example 130 kHz for monochrome ¼ VGA 0.5 frames/s using mpeg4compression. Dependent on requirements and compromises possible andacceptable in particular applications, other still more efficientencoding methods known in the literature may be used instead. Techniquesfor allowing video images to be sent between mobile units are known inthe art and so will not be described in detail.

In each of the systems of FIGS. 6 to 9, electromagnetic signals are usedto communicate between submerged stations. However, as will beappreciated, under water this provides a relatively short range, and soone of a group of mobile stations may not be able to communicatedirectly to another. To extend the usable range, a mesh network may beused to link between individual nodes. In this, each hop is anachievable shorter range. Repeaters extend range within a distributedad-hoc network (mesh networking). FIG. 10 illustrates an example ofthis. This includes mobile units A and C and a mesh transceiver B.Although not shown, the mesh would typically include a plurality of suchtransceivers B. Each transceiver of the mesh has a magnetically coupledelectrically insulated antenna and each communicates with others usingshort-range propagation. In use, rather than signals being sent directlybetween mobile diver units, they are sent via the intermediate meshtransceiver B. Hence, in this case communication between A and C ispossible through relay B.

In certain applications of the present invention, surface repeater buoysare deployed to link between buoys to extended underwater transmissionranges. The surface repeater buoy acts to receive the subsea radiosignal and then relays the information to a second buoy usingterrestrial radio devices, including but not limited to the use of ahigher carrier frequency.

The present invention can convey a variety of different types ofinformation, including but not limited to the following examples in thefields of audio transmission, video transmission, text datatransmission, control data, and other forms of information that can beencoded in digital form:

In one embodiment, audio transmission includes: diver to diver voice;diver to vessel voice; diver to shore voice; voice using a remotemicrophone and/or hydrophone; voice between submarine and submarine;voice between submarine and surface vessel; and voice between submarineand a shore station.

In another embodiment, video transmission includes: video imagesconveyed for assistance in steering and control of autonomous underwatervehicles (AUV) and remotely operated vehicles (ROV); images formonitoring underwater construction sites; images which assist thedocking of AUVs and other mobile underwater devices; images for findingand checking the presence of underwater objects; images for assessingdamage and the maintenance condition of varied underwater systems andplant such as pipelines, risers, valves and platforms; images forassisting the remote control of tools and manipulators on AUVs and ROVs;and images conveyed for assessment of environmental damage.

In another embodiment, text data includes: as required in diver to divercommunication; and in diver to shore or vessel communication.

In another embodiment, control data includes: data used as part of thecommunications aspect of supervisory control and acquisition systems(SCADA); and of command and control systems for underwater vesselsincluding backup for wired control.

Additional forms of information transfer can be utilized with thesystems and methods of the present invention. These additional forms ofinformation transfer include but are not limited to, data transmittedfrom/to seismic, geophysical, environmental and other underwater dataloggers and a surface vessel or an AUV or ROV; data of generic typestransferred between an AUV and a surface vessel or docking station; datatransferred from sensors to a vessel or shore for monitoring of theenvironment and for detection of the presence of alien objects as partof homeland security; data and communication control protocols requiredfor provision of internet and other communications access points inswimming pools, reservoirs, sea water areas such as around shipwrecksites; data communicated through ice, both solid and floating; data overcommunication links in the networking of sensors, assets, vehicles andpeople, not all of which are necessarily underwater; data transferredfor targeting and priming to/from a torpedo or other vehicle while in atorpedo bay or in motion, and the like.

To provide an even greater communication range, signals may be sent fromone underwater transceiver to another via an above-water air path. FIG.11 illustrates an example of this. In this case, underwater radiostations 120 are each provided with an underwater transceiver 122 thatis able to communicate with other underwater transceivers 124 that areconnected to, for example, buoys 126. As before, all of the underwatertransceivers 122, 124 have electrically insulated magnetic coupledantennas for allowing underwater near field electromagnetic propagation.Associated with each buoy 126 is a radio transceiver 128 that is able tocommunicate with the underwater receiver and also to transmit andreceive electromagnetic signal via an air path. Hence, the buoys 126 caneffectively act as air transmission repeater relays for forwarding viaan air path messages or signals originally generated underwater. In thisway, signals can be sent between underwater units or between anunderwater unit and a remote station over relatively long distances.This provides the possibility of long range, low data rate, low carrierfrequency telemetry and remote control from underwater or surfaceequipment. Of course, as will be appreciated, rather than sendingsignals from an underwater transmitter to an underwater receiver via asurface based relays, data could merely be transmitted from anunderwater transmitter to a surface based receiver, or vice versa. Thisis possible because the magnetic component of any electromagnetic signalcrosses the water to air boundary with relatively low attenuation.Magnetic coupled antennas make optimal use of this property. In similarmanner, data may be transmitted between an underwater site and anotherunderwater site or a site on the surface or on land, using for part orall of the route a path of lesser attenuation through the sea bed.

FIG. 12 illustrates an autonomous underwater vehicle 130 that includesthe transceiver 132 of FIG. 1. In this embodiment, the magnetic coupledantenna 134 is provided in the vehicle hull. Associated with the vehicle130 is a docking station 136 that includes a corresponding transceiver138. The vehicle and station transceivers 132 and 138 are arranged totransfer data using near field electromagnetic propagation. In practice,this means that data transfer only happens when the vehicle 130 and thestation 136 are relatively close together, for example less than abouttwo metres apart. This provides a very short range, but high bandwidthdata link that allows high data transfer rates. For example, operatingat a 70 MHz carrier a 10 Mbps data link could be operated over a rangeof 0.5 m.

To offer improved resilience, a modulation scheme can be used in thesystems described whereby the carrier is modulated by multiple narrowband sub-carriers, as shown in FIG. 13. The modulation scheme may bebased on orthogonal frequency division multiplexing (OFDM). The path forelectromagnetic radiation exhibits dispersion such that differentfrequencies of modulation propagate at different speeds and areattenuated by differing amounts resulting in distortion of the signal atthe receiving system. This effect is accentuated where a wide bandmodulation is used thus dispersion is more significant. In addition, thepath may exhibit some degree of multi-path characteristic where, otherthan the direct path between the communication stations, the energy maytake alternative paths either reflected from objects such as the seabed,sea surface or offshore structures or conducted through either the airor the seabed. Modulating the signals using multiple narrow bandsub-carriers allows distortions caused by the underwater transmissionpath to be overcome. At the transmitter, a single data stream can bemultiplexed to form many separate channels of data. Each narrowbandwidth channel is transmitted over a separate frequency channel. Eachchannel experiences different levels of propagation delay due to itsfrequency of transmission. At the receiver, the differential delay ofeach channel is removed and the multiple data streams re-assembled toextract the original broadband data stream.

Where modulation is used, at each transmitter the signal is modulatedonto multiple narrow band sub carriers to make up the required signalbandwidth. Many suitable modulations schemes are known such asquadrature amplitude modulation (QAM) and Orthogonal Frequency DivisionMultiplexing (OFDM). The combined signal is then modulated onto thecarrier. At the receivers, the signal is detected and split into thesame multiple narrow band carriers, which are in turn demodulated torecover the information. The processing may be analogue or digital,although typically the processing will be digital. The digitalimplementation could employ an inverse fast Fourier Transform (FFT) toform the multiple narrow-band signals into a single carriertransmission. This may be combined with an error correction codingscheme whereby redundancy is introduced to the digital bit stream toallow detection and recovery from corruption of the signal.

FIG. 14 illustrates another technique that may be employed with thepresent invention. This involves the use of guard bands between symbols,so that signals resulting from multiple paths do not interfere withadjacent symbols. This is possible because the symbol rates required oneach frequency are low compared to the overall data rate of the system.For example, a system transmitting 1 million symbols per second mayemploy 1000 frequency channels, this being made possible by the use ofdigital processing techniques such as FFTs. The individual symbol ratefor each channel is 1 thousand symbols per second, which is equivalentto 1 ms per symbol. Over a 1 km range, the multi-path effects willtypically be less than 30 □s so the guard band need only extend thesymbol length by less than 4%.

It will be appreciated, many types of modulation may be adopted singlyor in combination with various embodiments of the present invention,whether combined with OFDM or not, including but not limited to:quadrature amplitude modulation (QAM) with many possible constellationsknown in the art; phase modulation (PM); frequency modulation (FM) orphase shift keying (PSK); frequency shift keying (FSK); amplitudemodulation (AM); and amplitude modulation with single sidebandsuppressed carrier (SSB-SC), double sideband suppressed carrier(DSB-SC), single sideband with vestigial carrier, and the like.

FIG. 15 shows a system for allowing water-based vehicles to communicateat the surface, in particular where wave wash impinges the antennasystems. This has two stations 140, for example two mobile stations, onehaving the transmitter 142 of FIG. 2 and the other having the receiver144 of FIG. 3, although each could in fact be a transceiver. Many marinesystems are required to operate both submerged and at the surface andsome are regularly awash with wave action, as shown in FIG. 14. Forexample, autonomous underwater vehicles operate much of the timesubmerged. This poses a problem for communications between the AUV andsurface vessels for example. Typical traditional antennas that employelectric field coupling such as dipoles are ineffective for the periodswhen partially or fully immersed in water. However, in accordance withthe present invention, the use of magnetically coupled antennas allowscommunication underwater, above water and when the system is awash.

To improve performance, automatic gain control may be used to cope withthe variation of signal strength caused by wave wash. Gain control canbe implemented by means of a control loop. For example the receivedsignal strength can be measured by developing a voltage across arectifying detector diode. Amplifier gain can be controlled in responseto measured signal strength to compensate for increased path loss duringwave wash of the antennas. Gain control may be applied at both thereceiver and the transmitter to provide additional dynamic range, thetransmitter power being controlled where a two-way link allows theshort-term signal path loss to be determined. This system will operatesatisfactorily where one or more of the communicating antennas is whollyor partially immersed in water.

In all of the communications systems of the present invention, describedabove, the operating signal carrier frequency will depend on theparticular application. The carrier frequency is selected as a functionof the data transfer rate and the distance over which transmission hasto occur. For example, for short-range communications where a high datarate is required, a relatively high frequency would be used, for exampleabove 1 MHz. In contrast for long-range communications where attenuationlosses are likely to be a problem, relatively low frequencies would beused, for example below 1 MHz, and in many cases below 100 kHz.

Another technique that may be applied in any of the underwatercommunications systems described above involves the use of an adaptivecarrier frequency based on range of operation. In this implementation,the carrier frequency employed to convey information is chosen tomaximize the information rate possible for the given signal path. Themost significant influence on the optimum frequency to choose will bethe range between the communicating systems. One implementation usesmultiple fixed frequencies that are known to all communicating stations.To first establish a connection, transmission commences on the lowestfrequency. Once communication is established, the systems may then adaptthe frequency of operation up and down to maximize data rate. This maybe performed based on the received signal strength. An alternativescheme employs the lowest frequency at all times to maintain timing andto communicate the main frequency being chosen to carry information.

The electromagnetic communication system, in which embodiments of theinvention is embodied, may be combined with acoustic communicationand/or with optical communication to provide enhanced capability.Whereas acoustic communications offer long-range capability they arelimited in terms of robust operation in noisy environments and can onlyoffer a limited bandwidth. The range of operation is limited withelectromagnetic communications but it is immune to acoustic noise andhas a wide bandwidth capability. By way of example a system of thepresent invention can include an acoustic modem and an underwaterelectromagnetic communications system as described above. The twosystems can be combined in a processing unit to select thecommunications path based on appropriate criteria. These criteria mayinclude factors such as measured error rates, range of operation,measured signal strength or required bandwidth. If very high bandwidthis required when the ends of the communication link are close enough toallow optical communication, this method similarly may be brought intooperation in preference to, or in addition to, electromagneticcommunication.

In various embodiments, the system of the present invention includesenhancements to receive signal strength and/or communication distance.These enhancements can apply variously to transmitters, receivers andantennas, and are known in the art.

Directional antennas may be adopted to concentrate and maximize thepower which a transmitter sends in the direction of a receiver and, bythe principle of reciprocity, which a directional receive antenna canintercept. In as much as directional properties can be improved,communication range will be increased. If transmit and/or receiveantennas are steered towards each other, preferably with dynamicreal-time adjustment, then the optimum signal can be provided at alltimes. Diversity techniques employing multiple antennas at receiveand/or transmit sites may be adopted, and intelligent switching adoptedto use the most advantageous signal path at any time.

It will be appreciated that magnetic coupled antennas at the transmitterand receiver need not be of the same size. Where an end of thecommunications link is static or may be moved only occasionally, it maybe possible to deploy an unusually large antenna loop or solenoid. Forexample, this may be possible for an underwater fixed sensor, where itsantenna could lie flat on the sea floor; and for a static centralcommunication site. Such antennas could be formed of loops many metresin diameter if necessary. Whether deployed by the transmitter orreceiver, larger antenna size will increase the received signal e.m.f.approximately in proportion to the increased area of the antenna. Ofcourse, the largest possible size of antenna at both ends is usuallyadvantageous to maximize the received signal.

The magnetic and electromagnetic field from a transmitter (andcorrespondingly a receiver) may be increased by using latest magneticcore materials of the highest possible permeability in the antenna inorder to increase magnetic flux for given antenna dimensions.

While magnetic coupled antennas may be used, electromagnetic antennas ofplain wire similar to those of conventional radio methods, and electricantennas which predominantly excite and detect an electric field, canalso be deployed; and they may be deployed in combination to achieve thestrongest aggregate received signal.

For maximum signal from a magnetic coupled transmit antenna, thegreatest possible current is required in the loop or solenoid. Wherecryogenic cooling is possible, the use of superconductivity can increasethe current possible through the conductors of the antenna. In addition,highly sensitive receivers may be constructed using SQUID techniques andby using Josephson junction methods, as will be known to those skilledin these arts.

Particularly in deployment environments where receivers and/ortransmitters are powered by batteries of limited capacity and/ortransmission is of high power, methods are desirable to conserve energy.To achieve this, it is possible to transmit only when new relevant datais available at the transmit site; or to transmit only periodically; orto transmit only when a signal from the receive end requests data.Moreover, if the receiver knows when to expect data, either becausetransmission times are known or because the receive site requeststransmission, most of the receiver circuits may be dormant at othertimes and so conserve energy also. Energy may also be conserved byreducing transmit power to the lowest level necessary for reliablecommunication. Reduction from maximum power could be based on knownfactors such as distance, or the receiver could inform the transmitterdynamically of the level it is receiving so that, when possible, thetransmitter can reduce its power to a lesser level which is stilladequate or, conversely, increase power when necessary. If acousticand/or optical communication methods are available in addition toelectromagnetic, then it may be advantageous to switch to whichevermethod uses the least power for the communication conditions found to beencountered.

In certain embodiments, several antennas can be used with systems of thepresent invention and be deployed across a region of sea or sea bottomwhere divers or underwater autonomous vehicles are to operate, soincreasing the area over which communication can take place. Whilediscrete loop antennas will often be optimal, other forms of antenna mayalso be used including antennas of distributed wire or cable. Althoughthese may provide a weaker signal at a given distance from the wire,operation over a larger distance close to the antenna will be possible.The antennas can transmit and/or receive the same signal, or they couldhandle different signals.

Where different transmit signals are used, these also could supplylocation information to a diver or vehicle in each vicinity. Inaddition, multiple antennas could carry multiple channels, allowingmultiple links to operate simultaneously. By use of data multiplexing,as is known in the art, a number of logical channels may be carried bythe same carrier, and to different end stations or to differentfunctions at the same end station.

It will be appreciated that the embodiments illustrated in FIGS. 1-15can be used singularly as well as in various combinations of theembodiments. The systems and methods of the present invention can beused in seawater, fresh water and any brackish composition in between.Because relatively pure fresh water environments exhibit differentelectromagnetic propagation properties from seawater, differentoperating conditions may be needed in different environments.Optimization for specific saline constitutions will be apparent to anypractitioner skilled in this area.

While the invention is susceptible to various modifications andalternative constructions, certain illustrated embodiments thereof areshown in the drawings and have been described above in detail. It shouldbe understood, however, that there is no intention to limit theinvention to the specific form or forms disclosed, but on the contrary,the intention is to cover all modifications, alternative constructions,and equivalents falling within the spirit and scope of the invention.

1. An underwater remote sensing system comprising a transmitter fortransmitting an electromagnetic signal, a receiver for receiving anelectromagnetic signal reflected from an object and determining meansfor determining the location of the object using the received signal,wherein at least one of the transmitter and receiver is underwater. 2.An underwater remote sensing system as claimed in claim 1 wherein thedetermining means are operable to determine the location of the objectusing the field strength at three or more receiver positions.
 3. Anunderwater remote sensing system as claimed in claim 2 wherein three ormore receiver antennas are provided and the determining means areoperable to determine the location of the object by comparison of thefield strength of the signal received at each antenna.
 4. An underwaterremote sensing system as claimed in claim 2 wherein a single receiverantenna is provided and the determining means are operable to determinethe location of the object by comparison of the field strength of thesignal received at three or more different receiver locations.
 5. Anunderwater remote sensing system as claimed in claim 1 wherein at leastone of the receiver and transmitter has a magnetic coupled antenna. 6.An underwater remote sensing system as claimed in claim 1 wherein atleast one of the receiver and transmitter has an electrically insulatedantenna.
 7. An underwater remote sensing system as claimed in claim 6wherein each insulated antenna is surrounded by an impedance-matched lowconductivity medium, for example distilled water.
 8. An underwaterremote sensing system as claimed in claim 1 wherein the receiver antennais a directional antenna and the means for determining are operable touse the directionality of the receiver to determine the relativedirectional position of the reflecting target.
 9. An underwater remotesensing system as claimed in claim 1 wherein means are provided forvarying the frequency of the transmitter signal.
 10. An underwaterremote sensing system as claimed in claim 1 wherein the transmitter andreceiver are operable to implement a communications link.
 11. Anunderwater remote sensing system as claimed in claim 1 wherein thereceiver shares an antenna with the transmitter.
 12. An underwaterremote sensing system as claimed in claim 1 wherein the receiver isremotely located from the transmitter.
 13. An underwater remote sensingsystem as claimed in claim 1 wherein the transmitter and receiver aresubstantially co located.
 14. An underwater remote sensing system asclaimed in claim 1 wherein the location of the object is determinedrelative to at least one of the receiver, the transmitter or apre-determined reference.
 15. An underwater remote sensing system asclaimed in claim 1 wherein the transmitted signal has a frequency ofbelow 3 MHz.
 16. An underwater remote sensing system as claimed in claim15 wherein the transmitted signal has a frequency in the range of 1 MHzto 3 MHz.
 17. A system for determining underwater electromagnetic signalpropagation direction comprising at least one receiver for receiving theelectromagnetic signal and determining means for determining thedirection of propagation using field strength at three or more receiverlocations.
 18. A system as claimed in claim 17 wherein three or morereceiver antennas are provided and the determining means are operable todetermine the location of the object by comparison of the field strengthreceived at each antenna.
 19. A system as claimed in claim 17 wherein asingle receiver antenna is provided and the determining means areoperable to determine the location of the object by comparison of thefield strength received at three or more different receiver locations.